Memory cells, memory cell programming methods, memory cell reading methods, memory cell operating methods, and memory devices

ABSTRACT

Embodiments disclosed include memory cell operating methods, memory cell programming methods, memory cell reading methods, memory cells, and memory devices. In one embodiment, a memory cell includes a wordline, a first bitline, a second bitline, and a memory element. The memory element is electrically connected to the wordline and selectively electrically connected to the first bitline and the second bitline. The memory element stores information via a resistive state of the memory element. The memory cell is configured to convey the resistive state of the memory element via either a first current flowing from the first bitline through the memory element to the wordline or a second current flowing from the wordline through the memory element to the second bitline.

RELATED PATENT DATA Cross Reference to Related Application

This application is a continuation of U.S. patent application Ser. No. 12/833,810 which was filed on Jul. 9, 2010, now U.S. Pat. No. 8,154,906 which was issued Apr. 10, 2012, which is a continuation of U.S. patent application Ser. No. 12/014,232 which was filed on Jan. 15, 2008, now U.S. Pat. No. 7,768,812, which issued Aug. 3, 2010, the teachings of which are incorporated herein by reference.

TECHNICAL FIELD

Embodiments disclosed herein pertain to memory cells, memory cell programming methods, memory cell reading methods, memory cell operating methods, and memory devices.

BACKGROUND

Resistive random access memories may use a material capable of being configured in one of two different resistive states to store information. When configured in one of the resistive states, the material may have a high resistance to electrical current. In contrast, when configured in the other resistive state, the material may have a low resistance to electrical current. The resistive state in which the material is configured may be changed using electrical signals. For example, if the material is in a high-resistance state, the material may be configured to be in a low-resistance state by applying a voltage across the material.

The resistive state may be persistent. For example, once configured in a resistive state, the material may stay in the resistive state even if neither a current nor a voltage is applied to the material. Furthermore, the configuration of the material may be repeatedly changed from the high resistance state to the low resistance state or from the low resistance state to the high resistance state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart illustrating voltage/current relationships.

FIG. 2 is chart illustrating other voltage/current relationships.

FIG. 3 is a schematic diagram of a memory cell.

FIG. 4 is a schematic diagram of a memory device.

FIG. 5 is a schematic diagram of a memory device illustrating a current.

FIG. 6 is a schematic diagram of a memory device illustrating another current.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Embodiments of the invention encompass memory cell operating methods, memory cell programming methods, memory cell reading methods, memory cells, and memory devices. Example embodiments of such methods, memory cells, and memory devices are described with reference to FIGS. 1-6.

Random access memories may use resistive states of a memory element to store one or more bits of information. For example, a memory element capable of being configured in either a high-resistance state or a low-resistance state may store one bit of information by associating a bit value of “1” with the low-resistance state and a bit value of “0” with the high-resistance state. Alternatively, a bit value of “1” may be associated with the high-resistance state and a bit value of “0” may be associated with the low-resistance state.

The memory element may include a bipolar memory material. A positive voltage applied across the bipolar memory material may change the configuration of the bipolar memory material from a high-resistance state to a low-resistance state. Furthermore, a negative voltage applied across the bipolar memory material may change the configuration of the bipolar memory material from a low-resistance state to a high-resistance state.

Alternatively, a negative voltage applied across the bipolar memory material may change the configuration of the bipolar memory material from a high-resistance state to a low-resistance state and a positive voltage applied across the bipolar memory material may change the configuration of the bipolar memory material from a low-resistance state to a high-resistance. Accordingly, a bipolar memory material may be configured in a first resistive state using a voltage having a first polarity and may be configured in a second resistive state using a voltage having a polarity opposite that of the first polarity.

Examples of bipolar memory material include ionic conducting chalcogenides, binary metal oxides, perovskite oxides, colossal magnetoresistives, and polymers. Example ionic conducting chalcogenides that may be used as bipolar memory material include GeS, GeSe and Ag or Cu-doped GeS and GeSe. Example binary metal oxides that may be used as bipolar memory material include HfO_(x), Nb₂O₅, Al₂O₃, WO_(x), Ta₂O₅, TiO_(x), ZrO_(x), Cu_(x)O, and NiO_(x). Example ionic perovskite oxides that may be used as bipolar memory material include doped or undoped SrTiO₃, SrZrO₃, BaTiO₃.

Example colossal magnetoresistives that may be used as bipolar memory material include Pr_(1-x)Ca_(x)MnO₃ (PCMO), La_(1-x)Ca_(x)MnO₃ (LCMO), and Ba_(1-x)Sr_(x)TiO₃. Example polymers that may be used as bipolar memory material include Bengala Rose, AlQ₃Ag, Cu-TCNQ, DDQ, TAPA, and fluorescine-based polymers. Of course, other materials may be used as bipolar memory material. The materials listed above are provided by way of example rather than as an exhaustive list of bipolar memory material.

Referring to FIG. 1, a chart 100 depicts one example of a relationship 102 between voltages applied to a memory element initially in a high-resistance state and currents through the memory element resulting from the voltages. The memory element may comprise one or more of the bipolar memory materials described above. Chart 100 illustrates that as voltages applied to the memory element increase from −0.6 V to 0.2 V, no current, or a negligible amount of current, for example less than 1 micro amp, flows through the memory element. However, at a voltage substantially equal to a turn-on voltage 104, the memory element begins conducting current. As the voltage across the memory element is increased beyond turn-on voltage 104, the amount of current conducted by the memory element increases with the voltage without current clamping. The flattening of the current with voltage depicted in FIG. 1 results from current clamping of a measurement circuit.

Thus, chart 100 illustrates a change in the resistive state of the memory element. Initially, the memory element is in a high-resistance state as evidenced by the fact that the memory element does not conduct current or conducts a negligible amount of current when voltages smaller than the turn-on voltage are applied to the memory element. In this high-resistance state, the memory element may have a resistance of about or above 10⁹ ohms. However, once a voltage greater than or equal to turn-on voltage 104 is applied to the memory element, the memory element configures itself in a low-resistance state as evidenced by the fact that the memory element begins conducting current. In this low-resistance state, the memory element may be highly conductive and have a resistance of about a few thousand ohms.

Referring to FIG. 2, a chart 200 depicts one example of a relationship 202 between voltages applied to a memory element initially in the low-resistance state described above and currents running through the memory element resulting from the voltages. Chart 200 illustrates that currents resulting from voltages greater than about 0.25 V are substantially the same, which results from current clamping of a measurement circuit. Without the current clamping, the current increases with voltage. As the voltage is decreased below about 0.25 V, the current through the memory element accordingly decreases. As the voltage applied across the memory element becomes negative, the current through the memory element is also negative. However, when the voltage applied across the memory element is substantially equal to a turn-off voltage 204, the amount of current through the memory element is substantially zero. As the voltage is further decreased below the turn-off voltage, the current through the memory element remains substantially zero.

Thus, chart 200 illustrates a change in resistive state of the memory element. Initially, the memory element is in the low-resistance state as evidenced by the fact that the memory element conducts current when voltages greater than turn-off voltage 204 are applied to the memory element. However, once a voltage less than or equal to turn-off voltage 204 is applied to the memory element, the memory element configures itself in the high-resistance state described above as evidenced by the fact that the memory element stops conducting current or conducts only a negligible amount of current.

In some cases, once configured in the high-resistance state, the memory element may remain in the high-resistance state as long as a voltage greater than or equal to turn-on voltage 104 is not applied to the memory element. The memory element may remain in the high-resistance state even if no voltage is applied to the memory element. Accordingly, the high-resistance state of the memory element may be described as non-volatile since the high-resistance state might not change over time as long as a voltage greater than or equal to turn-on voltage 104 is not applied to the memory element.

Similarly, in some cases, once configured in the low-resistance state, the memory element may remain in the low-resistance state as long as a voltage less than or equal to turn-off voltage 204 is not applied to the memory element. In fact, the memory element may remain in the low-resistance state even if no voltage is applied to the memory element. Accordingly, the low-resistance state of the memory element may also be described as non-volatile since the low-resistance state might not change over time as long as a voltage less than or equal to turn-off voltage 204 is not applied to the memory element.

Since the high-resistance state and the low-resistance state may be non-volatile, the memory element may be used to store one bit information. For example, a memory element may represent a bit value of “0” when configured in the high-resistance state and may represent a bit value of “1” when configured in the low-resistance state. Furthermore, the resistive state of the memory element may be repeatedly changed over time. Accordingly, the memory element may be in the high-resistance state representing a bit value of “0” at one moment in time and the memory element may be in a low-resistance state representing a bit value of “1” at another moment in time. Similarly, the representation of bit value with resistance states can be the opposite of that described above.

Referring to FIG. 3, a memory cell 300 is illustrated. Memory cell 300 includes a memory element 302 having electrodes 306 and 304. Memory element 302 may comprise a bipolar memory material such as one or more of the bipolar memory materials discussed above. Memory cell 300 also includes a wordline 308 and two bitlines 314 and 316. Electrode 304 of memory element 302 is connected to a wordline 308.

Bitline 314 may be selectively electrically connected to memory element 302. For example, a diode 310 may be connected to bitline 314 and to electrode 306. When diode 310 is forward biased (e.g., forward biased beyond a cut-in voltage of diode 310), diode 310 may conduct current from bitline 314 to memory element 302, thereby electrically connecting bitline 314 to memory element 302. In contrast, when diode 310 is not forward biased (e.g., when diode 310 is reverse biased), diode 310 may hinder current from flowing from memory element 302 to bitline 314 or from bitline 314 to memory element 302 so that memory element 302 is not electrically connected to bitline 314.

Similarly, bitline 316 may be selectively electrically connected to memory element 302. For example, a diode 312 may be connected to bitline 316 and to electrode 306. When diode 312 is forward biased (e.g., forward biased beyond a cut-in voltage of diode 312), diode 312 may conduct current from memory element 302 to bitline 316 thereby electrically connecting memory element 302 to bitline 316. In contrast, when diode 312 is not forward biased (e.g., when diode 312 is reverse biased), diode 312 may hinder current from flowing from bitline 316 to memory element 302 or from memory element 302 to bitline 316 so that memory element 302 is not electrically connected to bitline 316.

In some configurations, a memory cell may comprise one or more devices that selectively electrically connect a memory element to a first bitline and/or a second bitline instead of diodes. For example, in one configuration, memory element 300 may use a first transistor in place of diode 310 and a second transistor in place of diode 312. When turned on, the first transistor may allow current to flow between bitline 314 and memory element 302, electrically connecting bitline 314 and memory element 302. When turned off, the first transistor may hinder current from flowing between bitline 314 and memory element 302 thereby electrically disconnecting bitline 314 and memory element 302.

Similarly, the second transistor may selectively electrically connect memory element 302 to bitline 316. Devices other than diodes or transistors may alternatively be used to selectively electrically connect memory element 302 to bitlines 314 and 316.

Memory cell 300 may store a bit of information via a resistive state of memory element 302. In one configuration, the bit may have a value of “0” or a value of “1.” For example, according to one convention, if memory element 302 is in a high-resistance state, the value of the bit stored by memory cell 300 may be “0” and if memory element 302 is in a low-resistance state, the value of the bit stored by memory cell 300 may be “1.” Of course, a convention in which a high-resistance state represents a bit value of “1” and a low-resistance state represents a bit value of “0” may alternatively be used.

A read operation may be used to determine a value of a bit stored by memory cell 300. According to one read operation, a first positive voltage may be applied between wordline 308 and bitline 316 so that wordline 308 is at a higher potential than bitline 316 and so that diode 312 is forward biased. The first positive voltage may be greater than a cut-in voltage of diode 312 but less than a sum of the cut-in voltage of diode 312 and the turn-off voltage (which was described above in relation to FIG. 2) of memory element 302 so that the resistive state of memory element 302 is not altered. A second positive voltage may be simultaneously applied between wordline 308 and bitline 314 so that wordline 308 is at a higher potential than bitline 314 and so that diode 310 is reverse biased. The second voltage may be lower than a breakdown voltage of diode 310. In some cases, the first voltage and the second voltage may be substantially the same voltage.

If memory element 302 is configured in a low-resistance state, current may flow from wordline 308 through memory element 302 and forward-biased diode 312 to bitline 316. Based on the current, a memory device comprising memory cell 300 may determine that memory element 302 is in a low-resistance state and therefore the value stored by memory cell 300 is a “1.” For example, the memory device may compare the current on bitline 316 with a reference current or the memory device may use the current on bitline 316 to create a voltage and may then compare the voltage with a reference voltage.

In contrast, if memory element 302 is configured in a high-resistance state, memory element 302 may hinder current from flowing from wordline 308 through memory element 302 and forward-biased diode 312 to bitline 316. Alternatively, memory element 302 may restrict an amount of current flowing from wordline 308 through memory element 302 and forward-biased diode 312 to bitline 316 to a negligible amount of current that may be clearly distinguished from an amount of current allowed to flow when memory element 302 is in the low-resistance state. Based on the lack of current, or the very small amount of current, the memory device comprising memory cell 300 may determine that memory element 302 is in the high-resistance state and therefore the value stored by memory cell 300 is a “0.”

Another method of reading memory cell 300 may alternatively be used. According to this method, a first positive voltage may be applied between bitline 314 and wordline 308 so that bitline 314 is at a higher potential than wordline 308 and so that diode 310 is forward biased. The first positive voltage may be greater than the cut-in voltage of diode 310 but less than a sum of the cut-in voltage of diode 310 and the turn-on voltage (which was described above in relation to FIG. 1) of memory element 302 so that the resistive state of memory element 302 is not altered. A second positive voltage may simultaneously be applied between bitline 316 and wordline 308 so that bitline 316 is at a higher potential than wordline 308 and so that diode 312 is reverse biased. The second voltage may be lower than a breakdown voltage of diode 312. In some cases, the first voltage and the second voltage may be substantially the same voltage.

If memory element 302 is configured in a low-resistance state, current may flow from bitline 314 through forward-biased diode 310 and memory element 302 to wordline 308. Based on the current on wordline 308, a memory device comprising memory cell 300 may determine that memory element 302 is in a low-resistance state and therefore the value stored by memory cell 300 is a “1.”

In contrast, if memory element 302 is configured in a high-resistance state, memory element 302 may hinder current from flowing from bitline 314 through forward-biased diode 310 and memory element 302 to wordline 308. Alternatively, memory element 302 may restrict an amount of current flowing from bitline 314 through forward-biased diode 310 and memory element 302 to wordline 308 to a negligible amount that may be clearly distinguished from an amount of current allowed to flow when memory element 302 is in the low-resistance state. Based on the lack of current, or the very small amount of current, the memory device comprising memory cell 300 may determine that memory element 302 is in the high-resistance state and therefore the value stored by memory cell 300 is a “0.”

In addition to reading a bit value from memory cell 300, a bit value may be written to memory cell 300. To write a bit value of “1” to memory cell 300, a first positive voltage may be applied between bitline 314 and wordline 308 so that bitline 314 is at a higher potential than wordline 308 and so that diode 310 is forward biased. The first positive voltage may be greater than a sum of the cut-in voltage of diode 310 and the turn-on voltage of memory element 302. If memory element 302 is in a high-resistance state, the first voltage (or a current resulting from the first voltage) may re-configure memory element 302 to be in a low-resistance state. If memory element 302 is already in a low-resistance state, memory element 302 may remain in the low-resistance state. Consequently, due to the first voltage, memory element 302 may be configured in a low-resistance state corresponding with a bit value of “1.”

A second positive voltage may be applied simultaneously with the first voltage. The second positive voltage may be applied between bitline 316 and wordline 308 so that bitline 316 is at a higher potential than wordline 308 and so that diode 312 is reverse biased. The second voltage may hinder current from flowing from bitline 314 to bitline 316. The second voltage may be lower than a breakdown voltage of diode 312.

The first voltage may result from a first voltage pulse and the second voltage may result from a second voltage pulse. In some cases, the first voltage and the second voltage may be substantially the same voltage.

Alternatively, a bit value of “0” may be written to memory cell 300. To write a bit value of “0” to memory cell 300, a first positive voltage may be applied between wordline 308 and bitline 316 so that wordline 308 is at a higher potential than bitline 316 and so that diode 312 is forward biased. The first positive voltage may be greater than a sum of the cut-in voltage of diode 312 and the turn-off voltage of memory element 302. If memory element 302 is in a low-resistance state, the first voltage (or a current resulting from the first voltage) may re-configure memory element 302 to be in a high-resistance state. If memory element 302 is already in a high-resistance state, memory element 302 may remain in the high-resistance state. Consequently, due to the first voltage, memory element 302 may be configured in a high-resistance state corresponding with a bit value of “0.”

A second positive voltage may be applied simultaneously with the first voltage. The second positive voltage may be applied between wordline 308 and bitline 314 so that wordline 308 is at a higher potential than bitline 314 and so that diode 310 is reverse biased. The second voltage may hinder current from flowing from bitline 316 to bitline 314. The second voltage may be lower than a breakdown voltage of diode 310.

The first voltage may result from a first voltage pulse and the second voltage may result from a second voltage pulse. In some cases, the first voltage and the second voltage may be substantially the same voltage.

The methods of writing a “0” and writing a “1” to memory cell 300 may be repeatedly used so that memory cell 300 stores different bit values over time. In some cases, memory element 302 may be re-written using these methods millions of times without damaging memory element 302. Since memory element 302 may remain in a resistive state without a voltage or current being applied to memory element 302 as was discussed above in relation to FIG. 1, memory element 302 may be said to preserve a bit value in a non-volatile manner. Accordingly, memory cell 300 may store a bit of information without having to be frequently refreshed or memory cell 300 may be refreshed at a rate lower than a rate used to refresh a volatile memory cell.

Referring to FIG. 4, a schematic diagram 400 of a portion of a memory device is illustrated. The memory device includes memory cell 300 as well as additional memory cells 402, 404, 406, 408, 410, 412, 414, and 416. The memory device may store a plurality of bits. For example, the memory device may store one bit in each memory cell of the memory device.

The memory cells of the memory device may be arranged to share bitlines and wordlines. In diagram 400, memory cells 402, 408, and 412 share wordline 418; memory cells 404, 300, and 414 share wordline 308; and memory cells 406, 410, and 416 share wordline 420. Furthermore, in diagram 400, memory cells 402, 404, and 406 share bitlines 424 and 426; memory cells 408, 300, and 410 share bitlines 314 and 316; and memory cells 412, 414, and 416 share bitlines 428 and 430.

Referring to FIG. 5, a schematic diagram 500 illustrating a configuration of the memory device of FIG. 4 is illustrated. The configuration may be used to write a bit value of “1” to memory cell 300 or to read a bit of information from memory cell 300. According to the configuration, a first positive voltage is applied across bitline 314 and wordline 308. A second positive voltage is applied across bitline 316 and wordline 308. Consequently, a current may flow from bitline 314 through memory cell 300 to wordline 308 as illustrated by arrow 502 and as was described above in relation to FIG. 3. If the first voltage is greater than a sum of the cut-in voltage of diode 310 and the turn-on voltage of memory element 302, a “1” may be written to memory cell 300 as was described above in relation to FIG. 3.

Alternatively, if the first voltage is greater than the cut-in voltage of diode 310 but less than a sum of the cut-in voltage of the diode and the turn-on voltage of memory element 302, the memory device may determine the value of a bit stored by memory cell 300 based on a current flowing from bitline 314 to wordline 308 as was described above in relation to FIG. 3.

It may be desirable to ensure that values stored by memory cells 402, 404, 406, 408, 410, 412, 414, and 416 are not disturbed while writing or reading memory cell 300. To avoid disturbances, bitlines and wordlines of the memory device may be configured with particular voltages.

For example, when writing a “1” into memory cell 300, bitlines 314 and 316 may be at a higher potential than wordline 308. Since memory cells 408 and 410 are also connected to bitlines 314 and 316, wordlines 418 and 420 may be configured to be at substantially the same potential as bitlines 314 and 316 to hinder current from flowing from bitline 314 and/or bitline 316 to wordline 418 and/or wordline 420.

Furthermore, it may be desirable to hinder currents from flowing from memory cells 404 and 414 onto wordline 308 so that a current on wordline 308 may be correctly attributed to memory cell 300. To do so, bitlines 424 and 428 may be configured to be at substantially the same potential as wordline 308 to hinder current from flowing from bitline 424 to wordline 308 via memory cell 404 and to hinder current from flowing from bitline 428 to wordline 308 via memory cell 414. In addition, bitlines 426 and 430 may be configured at a higher potential than wordline 308 so that current is hindered from flowing from bitline 426 through memory cell 404 to wordline 308 and from bitline 430 through memory cell 414 to wordline 308.

Other memory cells not connected to either wordline 308 or bitlines 314 and 316 (i.e., memory cells 402, 406, 412, and 416) may be configured to hinder consumption of current. For example, bitlines 424 and 426 and wordlines 418 and 420 may be configured with voltages causing the diodes of memory cells 402 and 406 to be reverse biased. Similarly, bitlines 428 and 430 and wordlines 418 and 420 may be configured with voltages causing the diodes of memory cells 412 and 416 to be reverse biased.

Referring to FIG. 6, a schematic diagram 600 illustrating a configuration of the memory device of FIG. 4 is illustrated. The configuration may be used to write a bit value of “0” to memory cell 300 or to read a bit of information from memory cell 300. According to the configuration, a first positive voltage is applied across wordline 308 and bitline 316. A second positive voltage is applied across wordline 308 and bitline 314. Consequently, a current may flow from wordline 308 through memory cell 300 to bitline 316 as indicated by arrow 602 and as was described above in relation to FIG. 3. If the first voltage is greater than a sum of the cut-in voltage of diode 312 and the turn-off voltage of memory element 302, a “0” may be written to memory cell 300 as was described above in relation to FIG. 3.

Alternatively, if the first voltage is greater than the cut-in voltage of diode 312 but less than a sum of the cut-in voltage of diode and the turn-off voltage of memory element 302, the memory device may determine the value of a bit stored by memory cell 300 based on a current flowing from wordline 308 to bitline 316 as was described above in relation to FIG. 3.

It may be desirable to ensure that values stored by memory cells 402, 404, 406, 408, 410, 412, 414, and 416 are not disturbed while writing or reading memory cell 300. To avoid disturbances, bitlines and wordlines of the memory device may be configured with particular voltages.

For example, when writing a “0” into memory cell 300, bitlines 314 and 316 may be at a lower potential than wordline 308. Since memory cells 408 and 410 are also connected to bitlines 314 and 316, wordlines 418 and 420 may be configured to be at substantially the same potential as bitlines 314 and 316 to hinder current from flowing from wordline 418 and/or wordline 420 to bitline 314 and/or bitline 316.

Furthermore, it may be desirable to hinder currents from flowing from wordline 308 into memory cells 404 and 414. To do so, bitlines 426 and 430 may be configured to be at substantially the same potential as wordline 308 to hinder current from flowing from wordline 308 to bitline 426 via memory cell 404 and to hinder current from flowing from wordline 308 to bitline 430 via memory cell 414. In addition, bitlines 424 and 428 may be configured at a lower potential than wordline 308 so that current is hindered from flowing from bitline 424 to wordline 308 through memory cell 404 and from bitline 428 to wordline 308 through memory cell 414.

Other memory cells not connected to either wordline 308 or bitlines 314 and 316 (i.e., memory cells 402, 406, 412, and 416) may be configured to hinder consumption of current as is illustrated in FIG. 6. For example, bitlines 424 and 428 may be configured at the same potential as wordlines 418 and 420 to hinder current from flowing through left-hand diodes of memory cells 402, 406, 412, and 416. Furthermore, bitlines 426 and 430 and wordlines 418 and 420 may be configured to reverse bias right-hand diodes of memory cells 402, 406, 412, and 416.

The above discussion has assumed that memory element 302 is configured so that memory element 302 changes to a low-resistance state when a voltage greater than the turn-on voltage of memory element 302 is applied between electrodes 306 and 304 so that electrode 306 is at a higher potential than electrode 304. Similarly, the above discussion has assumed that memory element 302 changes to a high-resistance state when a voltage greater than the turn-off voltage of memory element 302 is applied across electrodes 304 and 306 so that electrode 304 is at a higher potential than electrode 306.

However, memory element 302 may be reversed so that memory element 302 changes to a high-resistance state when a voltage greater than the turn-off voltage of memory element 302 is applied between electrodes 306 and 304 so that electrode 306 is at a higher potential than electrode 304. In this configuration, memory element 302 may change to a low-resistance state when a voltage greater than the turn-on voltage of memory element 302 is applied across electrodes 304 and 306 so that electrode 304 is at a higher potential than electrode 306.

Furthermore, the above discussion has assumed that a high-resistance state of memory element 302 corresponds to a bit value of “0” and that a low-resistance state of memory element 302 corresponds to a bit value of “1.” However, as was mentioned above, memory devices may be constructed based on an understanding that the high-resistance state of memory element 302 corresponds to a bit value of “1” and that the low-resistance state of memory element 302 corresponds to a bit value of “0” without changing the principles of writing and reading memory cell 300.

The above discussion has referred to a memory element having a high resistance state and a low resistance state. However, in some embodiments of the invention, a memory element may be configurable in more than two different resistive states. Such a memory element may store more than one bit of information and may be used in a memory cell such as memory cell 300. Each of a plurality of different programming voltages may correspond to a different one of a plurality of different resistive states of the memory element.

The methods of programming memory cell 300 described above may be adapted to program a memory element having more than one resistive state by applying one of the plurality of programming voltages to the memory element in order to configure the memory element in the resistive state corresponding to the applied programming voltage. Furthermore, the methods of reading memory cell 300 described above may be adapted to read the memory element by comparing a current resulting from a voltage applied to the memory element to a plurality of different reference currents to determine in which of the plurality of different resistive states the memory cell is configured.

In compliance with the statute, the subject matter disclosed herein has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the claims are not limited to the specific features shown and described, since the means herein disclosed comprise example embodiments. The claims are thus to be afforded full scope as literally worded, and to be appropriately interpreted in accordance with the doctrine of equivalents. 

The invention claimed is:
 1. A memory cell comprising: a memory element disposed to be selectively and reversibly configured in one of two or more different resistive states, the memory element comprising a first electrode and a second electrode; a first diode comprising an anode and a cathode, the cathode of the first diode being connected to the first electrode of the memory element; a second diode comprising an anode and a cathode, the anode of the second diode being connected to the first electrode of the memory element; a first bitline connected to the anode of the first diode; a second bitline connected to the cathode of the second diode; and a wordline connected to the second electrode of the memory element.
 2. The memory cell of claim 1 wherein the memory element comprises at least one of an ionic conducting chalcogenide, a binary metal oxide, a perovskite oxide, a colossal magnetoresistive, or a polymer.
 3. A memory cell programming method comprising: providing a memory cell comprising a first bitline, a second bitline, a wordline, and a memory element disposed to be selectively and reversibly configured in one of two different resistive states; configuring the memory element in a first one of the resistive states by conducting current from the first bitline through the memory element to the wordline while preventing current from flowing to the second bitline; and configuring the memory element in a second one of the resistive states by conducting current from the wordline through the memory element to the second bitline while preventing current from flowing to the first bitline.
 4. The method of claim 3 wherein the memory cell further comprises a first diode connected between the first bitline and the memory element and a second diode connected between the second bitline and the memory element.
 5. The method of claim 4 wherein preventing current from flowing to the second bitline comprises reverse biasing the second diode.
 6. The method of claim 4 wherein preventing current from flowing to the first bitline comprises reverse biasing the first diode.
 7. The method of claim 4 wherein a cathode of the first diode is connected to a first electrode of the memory element, an anode of the second diode is connected to the first electrode of the memory element, and a second electrode of the memory element is connected to the wordline.
 8. The method of claim 3 wherein conducting current from the first bitline through the memory element to the wordline comprises forward biasing the first diode.
 9. The method of claim 3 wherein conducting current from the wordline through the memory element to the second bitline comprises forward biasing the second diode.
 10. The method of claim 3 wherein the memory element is highly conductive of current when the memory element is in the first one of the resistive states and the memory element is highly resistive of current when the memory element is in the second one of the resistive states.
 11. A memory cell reading method comprising: providing a memory cell comprising a first bitline, a second bitline, a wordline, and a memory element disposed to be selectively and reversibly configured in one of a plurality of different resistive states; conducting current from the first bitline through the memory element to the wordline while preventing current from flowing to the second bitline; and based on the current, determining that the memory element is configured in a particular one of the plurality of different resistive states.
 12. The method of claim 11 wherein the memory cell further comprises a first selectively conducting device connected between the first bitline and a first electrode of the memory element and a second selectively conducting device connected between the second bitline and the first electrode of the memory element.
 13. The method of claim 12 wherein preventing current from flowing to the second bitline comprises disabling the second selectively conducting device and conducting current from the first bitline through the memory element to the wordline comprises enabling the first selectively conducting device.
 14. A memory cell reading method comprising: providing a memory cell comprising a first bitline, a second bitline, a wordline, and a memory element disposed to be selectively and reversibly configured in one of a plurality of different resistive states; conducting current from the wordline through the memory element to the second bitline while preventing current from flowing to the first bitline; and based on the current, determining that the memory element is configured in a particular one of the plurality of different resistive states.
 15. The method of claim 14 wherein the memory cell further comprises a first diode connected between the first bitline and the memory element and a second diode connected between the second bitline and the memory element.
 16. The method of claim 15 wherein preventing current from flowing to the first bitline comprises reverse biasing the first diode.
 17. The method of claim 15 wherein a cathode of the first diode is connected to a first electrode of the memory element, an anode of the second diode is connected to the first electrode of the memory element, and a second electrode of the memory element is connected to the wordline.
 18. The method of claim 15 wherein conducting current from the wordline through the memory element to the second bitline comprises forward biasing the second diode. 